Ceramic dielectric block filters offer several advantages over air-dielectric cavity filters. The blocks are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic block filter design, resonators are formed by cylindrical passages called through-holes which extend between opposed top and bottom surfaces of the block. The block is substantially plated with a conductive material (i.e., metallized) on all but one of its six (outer) sides and on the interior walls of the resonator through-holes.
The top surface is not fully metallized but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. In some designs, the pattern may extend to the sides of the block, where input/output electrodes are formed.
The reactive coupling between adjacent resonators is dictated, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern. These filters may also be equipped with an external metallic shield attached to and positioned across the open-circuited end of the block in order to minimize parasitic coupling between non-adjacent resonators and to achieve acceptable stopbands.
Although such RF signal filters have received widespread commercial acceptance since the 1970s, efforts at improvement on this basic design have continued to the present.
One such improvement has been the use of what is commonly referred to in the art as “capacitive cross-coupling” to increase the attenuation characteristics of a filter at frequencies below the passband thereof. An example of a filter incorporating a triplet capacitive cross-coupling design is disclosed in U.S. Pat. No. 6,559,735 to Vangala et al. in the form of a linear bypass electrode printed onto the top surface of the filter. This triplet cross-coupling design, however, cannot be used to place zeros to increase attenuation at frequencies above the passband of a ceramic monoblock filter inasmuch as the cross-coupling needs to be inductive (see, for example, “Cross-coupling in Microwave Bandpass Filters”, Microwave Journal, November 2004) and thus does not lend itself to practical implementation.
Moreover, in the triplet cross-coupling design of U.S. Pat. No. 6,559,735 increased attenuation below the passband is accomplished at the expense of attenuation above the passband. Although such skewed filter response is adequate for the consumer handset-related applications, it is not adequate for the cellular infrastructure base station-related applications where a more symmetrical response is desirable.
Still further, the cross-coupling triplet design precludes the use of ground-bars or notches between adjacent resonators, i.e., a feature which allows not only fine adjustments to inter-resonator coupling, as explained in U.S. Pat. No. 4,692,726 to Green et al., but also improves the overall out-of-band attenuation level by providing inductive coupling between resonators.
Therefore, the need continues for an improved RF filter which can offer improved attenuation on both the low and high sides of the passband while also making the filter response more symmetrical without increasing the filter size or cost of manufacturing. The present invention meets these and other needs.